"At about 770m (2,500ft) down, we came across some dark blocks - one was about the size of a beachball - but we couldn't figure out what it was," said Dr Marco Andreoli, an author on the Nature paper and a geologist at the South African Nuclear Energy Corporation and the University of Witwatersrand.

After chemical and mineral tests - which meant the material was cut up into smaller fragments - the scientists were astonished to find that the rock was a meteorite, a surviving relic from the collision.

When a large impactor strikes the Earth, a colossal amount of heat is produced; and the asteroid material is believed to vaporise or fuse with the surrounding rocks. A 10-km-diameter impactor is thought to generate temperatures of between 1,700-14,000C.

Consequently, scientists can only study these large impacts by looking at the chemical composition of material in the crater.

"What is amazing is that here we have these fragments - that may not have been attached to the asteroid, or maybe trailing behind it - that smashed into the Earth and survived the fiery furnace in the crater that formed; and then they got trapped," Dr Andreoli told the BBC News website.

"This is remarkable because this is something that people didn't think could happen."

New models

It meant, he said, that computer models of large impacts might now need to be revised, to take into account conditions where some of the asteroid material endures.

"Anything that helps scientists to model what happens when two bodies collide is good news."

The crater is hidden beneath the Kalahari Desert

Further investigation into the discovery has also revealed that the chemical composition of the space rock is slightly different to that of other meteorites that have been studied. It is a little more radioactive; there is more uranium, sodium, but less iron and nickel.

"All of our science of meteorites is based on meteorites that fell in the last few thousand years.

"But all of a sudden we can study a meteorite that fell 145 million years ago, and this opens the possibility that the nature of these impacting bodies has changed over the years," Dr Andreoli explained.

People in the UK can see fragments of the meteorite if they visit the Antenna Wing of London's Science Museum from Thursday.